1. Field of the Invention
The present invention relates to a thermal-compressive material and a thermal-compressive process, whereby the grain phenomenon issue is resolved. More particularly, the present invention relates to a thermal-compressive material and a thermal-compressive process for baseline of 0.09 um generation and beyond.
2. Description of the Prior Art
A typical photolithography process used in the fabrication of a semiconductor as follows. A material layer to be patterned is formed on a substrate. The material layer is made of polycide, polysilicon, silicon oxide, metal, nitride or the like. Then, a photoresist layer is deposited on the material layer to be patterned. Thereafter, the photoresist layer is exposed in selected regions with the light through a mask. The photoresist layer is then removed in the regions other than the selected regions. Then, a process can be performed in the underlying layer through the removed portions of the photoresist layer, such as removing the portions of the underlying layer to be patterned. As the variety of the suitable photoresist material changes, so does the variety of the wavelength of applied light.
There are three kinds of typical polymers for 193 nm resist. One is Acryle type that is famous for its etching resistance but it is bad for the photo performance. Another is Methacryle type. The advantage is the photo performance but not for the etching resistance. In order to compromise between the photo and the etching performance, the hybrid type is mixed by Acryle and Methacryle which has been introduced. However, the profile has a grain phenomenon issue. That is, the sidewall has a rough surface.
In the fabrication of semiconductor wafers, the photolithography steps are critical when patterning the minimum feature width, dictated by given photolithography equipment, onto a wafer. Several factors come into play that will affect the dimension and profile of a structure that has resulted from the photolithography steps performed.
One factor is the quality of the mask material (photoresist layer) itself. Another factor is the effectiveness of the light source (usually ultraviolet light) to expose the photoresist layer in direct correlation to an overlying mask or reticle. Though, the quality of photoresist layer must continually be monitored and improved, the exposure of the photoresist layer to a light source to provide the desired patterned, is an area where the major engineering efforts are ongoing.
The effectiveness of proper light exposure of the photoresist layer, not only depends on the photoresist material itself, but also on other factors such as the type of underlying film that is being patterned. Maintaining a desired profile becomes even more difficult when patterning a material that has a rough surface, such as a refractory metal that is made rough by the shape of its grains. For examples, when patterning a refractory metal, the unevenness of the grains results in the film possessing non-uniform adsorption and reflective properties to light. During a photo step, these non-uniform properties of the light which results from the light reflecting back into the photoresist layer at varying angles to cause reflective notching of the photoresist layer. Though reflective notching can be caused by any underlying film that is being patterned, it is a major problem when patterning the rough surfaced refractory metal.
Another challenge that is presented by the uneven grain of a refractory metal presents, comes to play during the etching step. Usually it is desired to obtain the most vertical profile as possible. However, the uneven grains of the refractory metal silicide, cause the vertical profile to become jagged and less vertical, both undesirable results.
Another factor of possessing non-uniform adsorption and reflective properties for the light is standing waves. Standing waves happen when the actinic light waves propagate through a resist film down to the substrate, where they are reflected back up through the resist. The reflected waves constructively and destructively interfere with the incident waves and create zones of high and low exposure with a separation of (λ\4n), where n is the index of refraction of the photoresist layer. It causes unwanted effects in resist layers in two ways. First, the periodic variation of light intensity in the resist layer causes the resist layer to receive non-uniform doses of energy throughout the layer thickness. The second effect is due to the variation of the total energy coupled to the resist layer by interference effects at different resist thickness. Both effects contribute to resolution loss of resist layer, and become significant as 1 μm resolutions are approached.
Referring to FIG. 1A and FIG. 1B, the phenomenon is shown in lateral view and top view. Shown in FIG. 1A is a material layer 12 to be patterned formed on a substrate 10. The material layer 12 to be patterned is made of polysilicon, polycide, silicon oxide, nitride or metal. A photoresist layer 14 is deposited on the material layer 12 to be patterned. Thereafter, the photoresist layer 14 is exposed in selected regions with the light 18 through a mask 16. In developing process, the photoresist layer 14 is then removed in the regions exposed. Shown in FIG. 1B is the result of developing the photoexposed photoresist layer 14 which is illustrated in FIG. 1A, wherein photoexposed patterned photoresist layers 14a and 14b formed upon the materials layer 12. Due to the standing wave and polymer grains, the photoexposed patterned photoresist layers 14a and 14b have irregularly formed rough sidewalls. The roughness forms numerous prominence 141 and indentations 142 on the surface of sidewalls. Shown in FIG. 1C is a CD-SEM of the noncircular profile for hole patterning appeared in positive defocus in top view although has in-spec CD. This is the aforementioned grain phenomenon issue.
Highly reflective substrates accentuate the standing wave effects, and thus attempts to suppress such effects have involved the use of dyes and anti-reflective coating below the resist layer. Other approaches have also been investigated, include: the use of incident UV radiation of multiple wavelengths, to reduce the variation of energy coupling by averaging; and post-baking between exposure and development. Although those approaches can resolve some problems, it is hard to completely eliminate the problem of grain phenomenon issue and needs many expensive cost and processes to archive them.
To be within acceptable manufacturing quality, critical dimensions (the widths of lines and spaces of critical circuit patterns) have to be within a predetermined acceptable margin for error tolerance. The size of the sidewall grains is probably larger than 10 nm or more and it is of little significance when critical dimensions are large in early processes. However, it is more and more critical for baseline of 90 nm generation and beyond. With grain phenomenon issue, critical dimensions are not uniform. For example, referring to FIG. 1C the hole pattern on photoresist is noncircular. For better manufacturing quality, the demand DOF (depth of focus) is limited. If the pattern transforming is more precise, the DOF and manufacturing quality can be improved both. What is needed is a solution to make the sidewall smooth, so that the profile is improved and more precise.